The present invention relates to a shadow mask used in a color cathode-ray tube and a manufacturing method therefor.
In general, a color cathode-ray tube is provided with a shadow mask that serves as color selecting means. The shadow mask is formed by integrally working a metal sheet that is relatively thin as a whole, and includes a curved-surface section in the form of a substantially spherical convex surface and a skirt section, which extends substantially at right angles to the curved-surface section and surrounds its whole periphery. The curved-surface section includes a perforated portion having a large number of electron beam apertures and a nonperforated peripheral edge portion on the outer periphery of the perforated portion.
Usually, the shadow mask of this type is manufactured by press-molding a flat mask that is composed of an initially flat metal sheet having the electron beam apertures. After the flat mask is first annealed so that it can be molded with ease, it is press-molded into a specified shape by means of a pressing mold. After the press-molding, the shadow mask surface is blackened so that an oxide film is formed thereon, whereupon the shadow mask is completed.
For various reasons, the thickness of shadow masks has recently been reduced to, for example, 0.12 to 0.13 mm or thereabout. As a result, the strength of the press-molded shadow masks is lowered, arousing a problem of deformation by an external impact.
Conventional press-molding is carried out in a manner such that a mask material is stretched in the surface direction by means of a mold, most commonly a punch mold, with a planished surface. Accordingly, stresses are concentrated on the perforated portion and peripheral edge portion of the shadow mask, so that the electron beam apertures are liable to suffer deformation called aperture elongation. Thus, the extent of plastic working of the shadow mask has its limit.
It is difficult, therefore, to work the whole perforated portion of the shadow mask uniformly. As a result, the mask inevitably includes local underworked portions, and is partially slackened or sagged. In this state, the whole shadow mask is not plastic yet, so that the molded mask cannot maintain its shape if it is dropped with an impact. The thinner the shadow mask, the more remarkable this effect is.
This problem can be solved by thickening the shadow mask. However, this solution is contradictory to the tendency toward thinner shadow masks, and makes it difficult to maintain the given shape of the electron beam apertures that are formed by etching.